This invention relates to a wear-resistant protective film for thermal printing heads.
Thermal printing heads are in extensive use with computers, word processors, facsimiles, and other similar devices. The head comprises a matrix of many dot resistance heating elements of polysilicon or the like, which can be selectively heated with an electric current to print data by thermal transfer onto paper through a printing ribbon. Since the paper is carried in sliding contact with the head, the contacting surface of the dot resistance heating elements must be protected with a highly wear-resistant protective film.
Each dot printing element, as a spot, of a thermal printing head, as illustrated in FIG. 1, comprises, from the base upward, a substrate 1 of alumina or the like, glaze glass 2 for heat accumulation, a heating unit layer 3 of polysilicon or the like, electrodes 4, 5, and a wear-resistant protective film 6. The numeral 7 indicates a heating spot.
The protective film 6 generally is required to have high hardness, reduced internal stresses attributable to heat and the composition and structure of its own, good resistance to wear, and stability against moisture, alkalies and other corrosive attacks. Diverse materials have hitherto been studied as candidates to meet these requirements.
Among the wear-resistant protective films of the prior art, those of the Si-O-N system introduced by Japanese Patent Application Public Disclosure Nos. 74177/1982 and 118273/1983 have great hardness and are superior in abrasion resistance, environmental resistance, and other properties required of thermal printing heads. Their disadvantages are the tendencies of cracking and separation from the base surface due to much internal stresses.
As noted above, the wear-resistant protective films of the Si-O-N system for thermal printing heads have the disadvantages of easily cracking and separating from the base under the influence of the internal stresses, in contrast with their otherwise superior properties as such. The present invention aims at overcoming those disadvantages of the wear-resistant protective films of the Si-O-N system for thermal heads.